/Engine/Source/Runtime/CoreUObject/Public/UObject/LinkerSave.h
Callback for arbitrary serializers to append data to the end of the ExportsArchive. Some PackageWriters used by SavePackage will write this data to a separate archive.
typedef [TUniqueFunction](API\Runtime\Core\Templates\TUniqueFunction)< void([FLinkerSave](API\Runtime\CoreUObject\UObject\FLinkerSave) &ExportsArchive, [FArchive](API\Runtime\Core\Serialization\FArchive) &DataArchive, int64 DataStartOffset)> AdditionalDataCallback